Typically, an electronic system includes a number of integrated circuit chips that communicate with one another to perform system applications. Often, the electronic system includes a controller, such as a micro-processor, and one or more memory chips, such as random access memory (RAM) chips. The controller communicates with the memory to store data and to read the stored data.
The RAM chips can be any suitable type of RAM, such as dynamic RAM (DRAM) including single data rate synchronous DRAM (SDR-SDRAM), double data rate SDRAM (DDR-SDRAM), graphics DDR-SDRAM (GDDR-SDRAM), low power SDR-SDRAM (LPSDR-SDRAM), and low power DDR-SDRAM (LPDDR-SDRAM). Also, the RAM chips can be any suitable generation of RAM, including double data rate two SDRAM (DDR2-SDRAM) and higher generations of RAM. Usually, each new generation of RAM operates at an increased clock speed and/or an increased data rate from the previous generation.
Sometimes, data and strobe signals are communicated between chips, such as a controller and RAM, to read and write data. To write data from the controller to the RAM, data and a clock or strobe signal are transmitted to the RAM and the received data is clocked into the RAM via the clock signal. To read data from the RAM, output data and a strobe signal are transmitted from the RAM. The output data and strobe signal are aligned to a clock signal via a delay locked loop (DLL).
Typically, the RAM receives an external clock signal and the DLL receives the external clock signal or an on-chip clock signal based on the external clock signal. The DLL provides an internal clock signal based on the external clock signal. The internal clock signal clocks the output data and strobe signal out of the RAM via output circuitry. The internal clock signal is fed back to a phase detector via a delay that mimics the delay of the output circuitry. The DLL aligns and locks the delayed internal clock signal to the external clock signal, which aligns the output data and strobe signal to the external clock signal. Since the external clock signal may drift over time and changes in the supply voltage and temperature may cause timing changes, the DLL runs continuously to maintain a lock state, which consumes considerable current.
In standby mode, integrated circuit chips are put into a low power state. If a DLL runs continuously in standby mode, the DLL is ready to drive output data as soon as the chip comes out of standby mode, however, considerable power is consumed in standby mode. If the DLL is switched off or loses lock state in standby mode, it takes considerable time to exit standby mode because the DLL must re-acquire lock state. As speeds increase and power consumption becomes more critical, these problems are amplified.
For these and other reasons there is a need for the present invention.